1. Field of the Invention
The present invention relates in general to an upper support used in a suspension system of a motor vehicle, for elastically connecting the body of the vehicle and a shock absorber of the suspension system, and more particularly to such an upper support which has significantly improved vibration-isolating capability.
2. Discussion of the Prior Art
In a conventional suspension system of a motor vehicle, a generally cylindrical upper support is usually interposed between the body of the vehicle, and a piston rod of a shock absorber which is linked with an axle or arm for supporting vehicle wheels. Such an upper support is adapted to prevent input vibrations received from the wheels through the shock absorber from being transmitted to the vehicle body, for example. The upper support usually includes a cylindrical inner rigid member to which the piston rod of the shock absorber is fixed, a cylindrical outer rigid member which is disposed radially outwardly of the inner rigid member and fixed to the vehicle body, and an elastic body interposed between the inner and outer rigid members for elastically connecting the two members.
The upper support of the above type is required to exhibit a relatively soft dynamic spring characteristic for improved vibration-isolating capability, for the purpose of preventing transmission of the vibrations from the shock absorber toward the vehicle body. At the same time, the elastic body of the upper support should have a high degree of stiffness so as to minimize the amount of deformation thereof against a static load applied thereto, for the purposes of avoiding changes in the attitude of the vehicle, and thus assuring high steering stability of the vehicle.
Namely, the upper support should provide a comparatively low dynamic spring constant for improved vibration-isolating capability, and a comparatively high static spring constant for assuring high steering stability of the vehicle. However, it is extremely difficult for the known upper support to fully satisfy the above requirements for the vibration-isolating capability and the steering stability, since the known upper support relies only upon the elastic deformation of the elastic body for isolating the input vibrations, and therefore cannot exhibit a sufficiently low dynamic spring constant together with a sufficiently high static spring constant.
In view of the above situation, there has been proposed a fluid-filled cylindrical upper support as disclosed in U.S. patent application Ser. No. 483,712 filed Feb. 23, 1990, which is assigned to the assignee of the present invention. The upper support disclosed therein has a fluid chamber filled with a non-compressible fluid and defined between the inner and outer rigid members which are elastically connected by the elastic body. The upper support further includes an annular resonance member, which is accommodated in the fluid chamber such that the resonance member radially extends from one of the inner and outer rigid members toward the other rigid member, so that the fluid chamber is divided into two sections located on the axially opposite sides of the resonance member. Upon application of a vibrational load between the inner and outer rigid members, pressures of the fluid in the two sections of the fluid chamber change relative to each other. Between a circumferential surface of the resonance member and an inner wall of the fluid chamber, there is formed a resonance portion which defines a fluid passage through which the fluid is forced to flow between the two sections upon application of the vibrational load. When the thus constructed upper support receives middle- to high-frequency vibrations to be isolated, the upper support provides a sufficiently lowered dynamic spring constant for isolating the vibrations, based on the resonance of the fluid flowing through the resonance portion, without significantly lowering the static spring constant of the upper support.
However, the upper support of the above type only exhibits a relatively low spring constant with respect to the vibrations having frequencies lower than the resonance frequency of the fluid flowing through the resonance portion as described above. When the upper support receives vibrations having frequencies higher than the resonance frequency, the flow resistance of the fluid passing through the resonance portion increases to such a high degree that the pressure in the two sections of the fluid chamber rises, resulting in an extremely increased dynamic spring constant of the upper support. For high-class luxury cars recently available on the market, in particular, the upper support is required to exhibit high isolating capability for vibrations having frequencies higher than the resonance frequency, so as to effectively reduce medium- to high-speed booming noises and road-oriented noises. Therefore, the upper support used in such high-class luxury cars has to avoid an extreme increase in its dynamic spring constant upon application of the vibrations having considerably high frequencies, while maintaining a low dynamic spring constant with respect to middle- to high-frequency vibrations, based on the resonance of the fluid flowing through the resonance portion.